1. Field of the Invention
The present invention relates to a time division adaptive retransmission technique for portable radiotelephones and, more particularly, to a radiotelephone system arrangement where the terminal and portable station each include at least two antennas capable of transmitting and receiving orthogonally polarized signals, and the technique used alternately transmits a preamble on each antenna at the beginning of each time division frame from the portable station to permit optimum antenna selection at both stations using a single receiver and transmitter in the portable station.
2. Description of the Prior Art
The reduction of fluctuations of portable radiotelephone signals caused by random orientation and multipath propagation has been of interest in recent years. A typical technique for overcoming fades has been to provide space diversity using two or more antennas and then selecting the antenna which provides the best signal to the base or portable station receivers at any instant of time, or only when the currently selected antenna signal indicates a fade below a predetermined threshold. In this regard see, for example, U.S. Pat. Nos. 3,693,088 issued to A. J. Rustako, Jr. et al on Sept. 19, 1972; 4,041,397 issued to P. W. Bauer et al on Aug. 9, 1977; and 4,317,218 issued to F. G. Perry on Feb. 23, 1982.
Another technique is the use of N-branch phase conjugate adaptive retransmission where each branch determines the phase of the received signal and uses that phase to retransmit its signal back to the remote location. In this regard see, for example, U.S. Pat. Nos. 3,631,494 issued to M. J. Gans et al on Dec. 28, 1971 and 3,696,421 issued to J. S. Bitler on Oct. 3, 1972, and the article "A Mobile Radio Single-Frequency `Two-Way` Diversity System Using Adaptive Retransmission from the Base" by J. S. Bilter et al in IEEE Transactions On Communications, Vol. COM-21, No. 11, November 1973 at pages 1241-1247.
Digital time-division adaptive retransmission in the mobile radio environment was also analyzed more recently in the article "A New Approach to High-Capacity Digital Mobile Radio" by P. S. Henry et al in BSTJ, Vol. 60, No. 8, October 1981 at pages 1891-1904.
The problem remaining in the prior art is to provide a technique using only one transmitter and receiver at the portable station which will substantially mitigate the effects of random orientation and multipath propagation for portable radiotelephones while selecting a best antenna of two or more antennas for transmission and reception.